All BESSY II undulators use permanent magnets based on the rare earth alloys Nd2Fe14B. This magnetic materials exhibit a magnetization of typically 1.2 Tesla corresponding to a surface current density of 1,300 A/mm. The high coercive force of these permanent magnet materials and a permeability close to 1 allow magnetic arrangements such as the APPLE II design to generate any desired polarization. There presently is no alternative to the use of permanent magnets: Normal conducting electromagnets not even get close to the current density of permanent magnets due to cooling limitations, while superconducting technology reaches even higher magnetic performance. However, so far there is no mature technology to reproducibly tune the magnetic field.

Permanent magnet undulators exist in two different designs: In pure permanent magnet undulators such as Halbach I undulator (linear polarization) or APPLE II (variable polarization) (link-16) design the field is only generated from blocks of permanent magnet. The principle of linear superposition of fields allows accurate prediction of the field. All elliptically polarized undulators of the UE type at BESSY employ this design. In so-called hybrid undulators high permeability pole pieces made from permendur (an alloy of composition Fe49Co49V2) channel the magnetic flux towards the midplane. The high homogeneity of the permendur material suppresses field errors. The magnetic performance is increased by shaping the poles and the magnets as wedges as is done in the linearly polarized U49 undulators and the U41 undulator.